1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device employing a wafer obtained by slicing a silicon single crystal grown by a Czochralski method. More specifically, the present invention relates to a method in which a denuded zone free from impurities of metal or the like and free of crystal defects is formed on the surface layer of a wafer, whereupon a semiconductor device is formed in the denuded zone.
2. Description of the Prior Art
A conventional method of forming a denuded zone in a surface layer of a silicon wafer comprises a heat treatment including from one to three steps utilizing oxygen precipitation. Such heat treatment methods are disclosed in, for example, "Intrinsic Gettering by Oxide Precipitate Induced Dislocations in Czochralski Si", by T.Y. Tan, App. Phys. Letters; vol. 30; No. 4; 1977, P. 175-P. 176; "A Study on Intrinsic Gettering in CZ Silicon Crystals: Evaluation, Thermal History Dependence, and Enhancement", by H. Tsuya et al., J. Electrochem. Soc., vol 129, No. 2, 1982, P. 374-P. 379; Japanese Patent Publication No. 52329/1983; Japanese Patent Publication No. 3375/1983; and Japanese Patent Laying Open Gazette No. 128037/1982.
In the following, the process of forming a denuded zone will be described by taking an example of a conventional method including two steps.
At the first step, a wafer is annealed at a relatively low temperature of 600.degree. C. to 800.degree. C. in an atmosphere of N.sub.2 gas (generally, N.sub.2 is used as an inert gas ) for a long period of, time of say, 8 to 45 hours, whereby excessive interstitial oxygen elements Oi are precipitated on the whole wafer. Then, in the second step, the wafer is annealed at the temperature of say 1000.degree. C. an atmosphere of N.sub.2 or O.sub.2 gas. At the second step the oxygen Oi is diffused outward in the wafer, whereby nuclei to become defects disappear, so that a denuded zone is formed, while crystal defects of high density are introduced inside from the denuded zone.
The denuded zone thus formed is utilized to form a semiconductor device of a high quality therein. The crystal defects of a high density in the inner layer also serve as a getter for metallic impurity, so that the function of the semiconductor is prevented from being degraded. The closer to the surface the inner crystal defects are (i.e.; the smaller the thickness of the denuded zone) the more effective is the gettering; however, if the inner crystal defects are formed to be too close to the surface, this increases the probability of the defects being even burning included in the device, thereby to adversely affect the device. Accordingly, it is important to control the thickness of the denuded zone and the density of the inner crystal defects serving as a getter.
However, in accordance with a conventional denuded zone forming method, the thickness of the denuded zone and the density of the inner crystal defects are influenced by an initial concentration of oxygen, existence of defects and the like in a silicon wafer. More specifically, even with a particular oxygen concentration, a denuded zone may be differently formed and, at times no denuded zone may be formed.